Influence of Dry and Wet High Energy Ball Milling On Reactivity In Ni-Al Gasless Heterogeneous System

The influence of short-term (up to 20 min) high energy ball milling (HEBM) on the reactivity of a heterogeneous Ni-Al system has been investigated. More specifically, we studied the effects of the dry and wet HEBM on the self-ignition temperature and kinetics of chemical reactions in the stoichiometric 1:1 mol Ni-Al mixtures. It was shown that HEBM under dry inert conditions leads to formation of coarse (0.1−2 mm) Ni/Al composite particles with defined layered metal microstructure. The electron microscopy investigation revealed that the thickness of metal (Ni, Al) layers in such composite particles varies from 0.1 to 1 µm. However, XRD analysis showed that no intermetallic phases were formed during the dry HEBM. In turn, the thermal analysis revealed that 20 min of dry HEBM leads to the significant decrease of the exothermic reaction onset temperature from ~660 ^oC, i.e. eutectics in Ni-Al system, to ~400 ^oC. For more enhancement of the reactivity, the complex approach has been used, i.e. the dry HEBM process was “arrested” just before the reaction initiated in the milling vial, followed by additional wet HEBM in hexane for 10min. Such approach decreases the size of Ni/Al composite particles to 30−40 µm, as well as leads to their microstructural transformation. Thermal analysis revealed that self-ignition temperature for such media is extremely low and equals to 300 ^oC. Apparent activation energies of different HEBM-formed reaction medium were also measured by using conventional differential thermal analysis, as well as unique, so-called, electro thermal explosion method. Both methods showed the significant influence of HEBM on the effective activation energy for the considered reactions. The mechanisms of the dry and wet HEBM’s influence on the chemical reactivity of the considered systems are also discussed.